CO2 laser cleaning of gum residues on architectural stone and quantitative assessment by hyperspectral imaging

Different methods have been developed to remove deposits or crusts on the architectonic elements located in the center of the cities, such as graffiti, patinas, gypsum black crusts, etc. Short pulsed lasers with wavelengths ranging from UV to NIR have demonstrated to be effective to clean some of these deposits or crusts. Here, we present an alternative laser cleaning method to remove chewing gum on architectural elements of natural stone, based on the use of a continuous wave, low power CO 2 laser emitting in its fundamental wavelength (10.6 μm). Chewing gums are mainly composed of their rubber or elastomer components which strongly absorb the CO 2 laser radiation. Thus, we demonstrate the effectiveness of this compact and robust type of laser to clean these substances, deposited on granite, without damaging the substrate. Additionally, we propose a method to quantitatively assess the cleaning level based on hyperspectral reflectance imaging. This method is based on the determination of the coefficients of abundance of granite (α G ) using reflectance curves obtained by a hyperspectral camera. The results obtained have been confirmed using characterization techniques commonly used in the cleaning level determination: stereomicroscopy, profilometry, Raman spectroscopy and Fourier-transform infrared spectroscopy. This non-invasive method of cleaning control will allow decisions to be made in real time during the cleaning process of architectonic heritage, avoiding over-cleaning or insufficient cleaning.